checkpatch.pl: add check for Change-Id

A commit hook for the Gerrit code review server [1] inserts change
identifiers so Gerrit can track patches through multiple revisions.
These identifiers are noise in the context of the upstream kernel.
(Many Gerrit servers are private.  Even given a public instance, given
only a Change-Id, one must guess which server a change was tracked on.
Patches submitted to the Linux kernel mailing lists should be able to
stand on their own.  If it's truly useful to reference code review on a
Gerrit server, a URL is a much clearer way to do so.) Thus, issue an
error when a Change-Id line is encountered before the Signed-off-by.

1. https://gerrit.googlesource.com/gerrit/+/master/gerrit-server/src/main/resources/com/google/gerrit/server/tools/root/hooks/commit-msg



Signed-off-by: Christopher Covington <cov@codeaurora.org>
Cc: Joe Perches <joe@perches.com>
Cc: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>